NREL Helps PV Industry Make Panels Last

June 6, 2012

Working in his lab at the NREL Outdoor Test Facility, NREL scientist Michael Kempe measures the "creep" of the top and bottom glass of a solar module, testing the encapsulant and demonstrating how enough stress can produce a spectacular failure. Credit: Dennis Schroeder

During 30 years on a rooftop, a solar panel gets bombarded by UV rays, soaked by rain, buffeted by wind, pounded by hail.

How well it stands up to that beating is a crucial factor in setting the warranties of solar modules — and in convincing the public that solar energy can be counted on like the sun rising in the east.

The U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) plays a crucial role in improving the reliability of the photovoltaic (PV) panels that are being installed on rooftops in record numbers.

NREL helps set standards for reliability and serves as a neutral third party in tests of manufacturers' new solders, edge seals, and glues. At its Golden, Colorado, campus, NREL subjects solar panels to heat, humidity, and mechanical stress to simulate conditions in Denver, Phoenix, the Philippines, and elsewhere.

In March, leading scientists and engineers in the industry gathered at NREL for the PV Module Reliability Workshop. The workshop encouraged a frank discussion of reliability problems that can plague solar power companies.

What standards are needed for the glue in the edges that seal a panel's top and bottom? How does weather affect cracking? What can be done to prevent one glass panel from creeping away from the other?

On what looks like a whirling see-through geodesic dome — albeit just two feet in diameter — NREL scientists attach matchbook-sized samples that simulate the construction of PV modules to determine at what combination of UV radiation, high temperature, high humidity, and mechanical stress those samples can fail.

It's important that manufacturers not just check for single stresses. By demonstrating that a combination of two or three factors can cause a failure, NREL is helping manufacturers prepare for the worst.

"We help manufacturers to know what kind of stress to put on their samples to determine if Sample A is better than Sample B," Kempe said. "Every tiny detail, every aspect of these things has to be examined."

A typical 12-millimeter-wide edge seal should keep out moisture anywhere in the world — from Salt Lake City to Bangkok — if it maintains a good adhesion, Kempe said. And the cost is between $1 and $2 a module, whether it is a tape-style edge seal or a hot-melt extrusion.

In the case of "creep," NREL's sophisticated tests showed that the problem isn't as big as was feared.

In a solar module, two pieces of glass are adhered together with a plastic encapsulant that may be solid at one temperature but flow — or "creep" — at another temperature. If it flows during the expected lifetime of a solar module, solar panel components can be displaced, and that can cause a short, break electrical connections, or even cause fires.

The stakes are high: a one-centimeter creep can expose live wires to the elements, and that can cause arcing or other serious safety problems.

NREL's tests found that most encapsulants used today or proposed for future use do a very good job of preventing creep. But showing that failure is possible keeps manufacturers from becoming complacent.

Last summer, Kempe and his colleagues used eight different encapsulants from six manufacturers to assemble several mock and actual solar modules. The scientists then evaluated them side by side in an objective manner, and in a way that uncovered strengths and weaknesses of the various encapsulants without pointing fingers at individual companies. The industry's trust in NREL made the tests possible. "They were able to participate without the fear of being singled out," Kempe said.

The researchers put insulating materials on the test modules and deployed them in Arizona so they would reach the highest temperatures (104°C) that are likely in the field.

The only material that crept significantly in the outdoor experiments was one that was intentionally formulated improperly so that it would still melt at moderate temperatures.

"All the other plastic materials that people in the industry were considering for encapsulation were essentially OK outdoors," Kempe said. "It would only be under very extreme circumstances that you might have a problem. The standards community realized that this stumbling block was not nearly as big an issue as was suspected."

Stress, Temperature Tests Help Prevent Cell Overheating

Shown here is a close-up of the inside of an Atlas CI 4000 Xenon Weather-Ometer used to test small samples of solar panels. NREL scientists apply temperature, humidity, and mechanical stress to the samples to show industry how they can fail.Credit: Dennis Schroeder

NREL also works on the problem of concentrating PV cells overheating in a module. Concentrating PV uses lenses to focus more sunlight on a solar cell. The solder or epoxy that adheres the panel's glass and edges will fatigue with time because of temperature changes that happen with the weather, NREL scientist Nick Bosco said. When the attachment goes bad, heat can't escape, and the cell overheats.

NREL uses high-frequency weather data to model the changes in cell temperature for Houston, Los Angeles, Albuquerque … wherever a company wants a climate test. The data are publicly available.

The most damaging locales are those with high temperatures and partly cloudy skies. The frequent temperature changes when clouds pass by can cause extra stress. "In Golden, Colorado [site of NREL's main campus], where we get hot mornings and then clouds roll in every afternoon, that can be more damaging than in Phoenix where you don't have many clouds," Bosco said. "We're early in the process, but we're seeing easily a 20% to 40% difference between certain locations."

To test the effect of temperature cycles on the modules, NREL uses various solders and epoxies to attach pieces of the panels, and then exposes them to different temperatures at varying intervals. Researchers test thermal cycling in indoor chambers and expose modules to outside conditions, comparing the results.

"We're interested in how cracks grow in the solder as the module goes through cycles," Bosco said. "Our instruments can image the cracks on a computer, analyze them, and measure their size. We'll do that periodically, then put the module back in the chamber, do more cycles, then measure the growth rate of the cracks as a function of the number of cycles."

Bosco is working on models and experiments to determine the amount of damage the attachment will accumulate. The goal is for the indoor test chamber to accurately reflect outdoor conditions.

"The amount of damage the attachment accumulates is different for every city, and we're hoping to model that," Bosco said. "We're hoping to be able to make real-life predictions based on location." So many cycles in the chamber is equal to so many years outside. "So, a company might expect similar crack growth after so many years."

The challenge for industry is to design solar modules that are very durable and reliable, yet not overly expensive. NREL scientists and their industry counterparts agree they can meet that challenge.

"They're looking for a route to a less expensive design and architecture of a cell assembly," Bosco said. NREL is able to figure out why a solution works, not just that it does work. It can report that a change in design or materials has this or that consequence in reliability. And NREL shares that knowledge with the industry to help the technology move forward.

NREL scientists and their industry partners have learned that an accelerated test will mean different things in different locations — and that the material and architecture of the design can influence reliability dramatically. "You can certainly have an expensive bad design," Bosco said. And, of course, a good product that is incorrectly installed can fail.

As tempting as it is to accelerate the testing so that new, presumably better products can get to market sooner, testing experts know that validating a product for 20 or 30 years of useful life is complicated without comparisons to real-life durability.

So, NREL and the industry keep a poultry analogy in mind. "When you're trying to hatch an egg, you give it 25 days at about 40°C, and you get a chicken," Kempe said. "If you try to accelerate the time by accelerating the temperature, you get a boiled egg."

The results of NREL's testing will provide the technical basis for changes to reliability standards.

Today, the standards aren't robust enough to predict the overall longevity of solar panels. NREL, the PV industry, and the attendees of the PV Reliability Workshop are working toward the day when tests and standards can determine the lifetime reliability of a module.

"What can come out of this is a graded test sequence," Bosco said. "If you pass, say, Level A, it means the module is good for a lifetime in these certain locations. A stricter Level B certification will provide a similar lifetime warranty in more damaging locations."

PV Reliability Workshop at NREL Set Ambitious Goals

The PV Reliability Workshop brought scientists and engineers together to discuss how to improve the reliability of solar panels.Credit: Dennis Schroeder

More than 160 people attended the Feb. 28–March 1 NREL PV Module Reliability Workshop in Golden, Colorado, and discussed, among other topics, keeping moisture out of PV modules, reliability issues specific to thin-film and PV concentrator modules, new test methods, performance failures, and modeling of reliability.

"Every company is required to share a presentation in order to attend the workshop. If everyone brings a little to share, everyone takes home a lot of information, and that leads to faster progress toward understanding how PV modules will fare as they are deployed worldwide," Sarah Kurtz, the NREL senior scientist who organized the workshop, said.

Task Groups reported ambitious goals, including:

Rewriting standards by December with regard to solder-bond quality control

Proposing new standards for tests on thermal and mechanical fatigue

Adding a test to better measure how humidity, temperature, and voltage can affect reliability

Further study to better ensure reliability of diodes

Discussing how UV exposure affects PV reliability in a range of environments, and how both UV and humidity interplay in a module's performance.

New Focus on Climate Effects

NREL's workshop this year included a new discussion about how to quantify and communicate the durability of PV modules for different climate zones and applications.

Will a module that lasts long in one location hold up equally well in other locations? Should a 30-year warranty for a solar panel be adjusted when the panel is sold in a very hot climate, or in tundra conditions, where sometimes the panels don't last as long?

First Solar recently announced plans to retain a 1% higher warranty reserve in recognition that more of its modules are being installed in hot climates.

Kurtz wasn't surprised by First Solar's move.

"It's not just how well modules do in Germany," Kurtz said. "The PV industry is selling to markets around the world, and manufacturers are recognizing that modules might last different amounts of time in different locations."

Trust Essential to Workshop's Success

"Durability is a continuing quest, and the scientists know there's always room for improvement, even when it's going in the right direction," NREL scientist Michael Kempe said. "There are always stumbling blocks, barriers, and plateaus to get past, and those at the workshop believe they can overcome them all."

Durability "is a very difficult topic for companies to talk about," Kempe said. "That's why we restrict attendance at the workshop to people who are more scientifically based — so you don't get lawyers, marketers, prospective buyers, or those looking for a competitive edge. Having this tighter group is good for opening up the discussion about durability issues."

"The PV Workshop was great," NREL scientist Nick Bosco said. "It was nice to have this semi-private platform where companies feel more comfortable to share their trials and tribulations. It's helpful for everybody to know the stumbling blocks and what we're doing to get over them."